Aircraft designers have for many years recognized the need for variable-geometry wings to provide good performance at both upper and lower portions of the aircraft speed range. In high-speed cruising flight, a small-area low-drag wing is needed for optimum performance and ride comfort. A highly loaded cruise-optimized wing of this type, however, is inconsistent with the need for production of high lift for safe operation at low speed during takeoff and landing. Designers accordingly provide aircraft with wings of variable area and contour for good low-speed short-field performance, while still being capable of achieving the desired characteristics during cruising flight.
Variable wing geometry is commonly achieved by providing hinged or otherwise movable flaps at the wing trailing edge. Plain or split flaps sometimes used on small aircraft in effect provide a variable-camber wing for improved low-speed operation. A significant further improvement in low-speed performance is offered by the well-known Fowler-type flap which, when extended, enables a significant increase in wing area and boundary layer control, in addition to the desired increase in wing camber during landing and takeoff.
A Fowler flap is a movable part of the undersurface of the wing trailing or aft portion, and the flap typically extends spanwise from the aircraft fuselage to roughly the midpoint of the wing length. When fully retracted, the flap smoothly completes the contour of a highly loaded wing which is optimized for efficient and comfortable high-speed flight. The flap is extended by being moved rearwardly and downwardly away from the trailing edge of the fixed wing to provide high lift and increased wing area during low-speed flight.
While the Fowler flap is a popular and effective way of achieving variable wing geometry, it has several deficiencies in conventional application. Each Fowler flap is typically provided with two or more fixed supporting and guiding tracks which extend aft from the wing, causing increased aerodynamic drag during cruising flight when the flap is retracted. Flap actuating mechanisms are expensive and complex in that Fowler flaps are usually driven by actuators at inboard and outboard positions, and sometimes by additional actuators at intermediate positions. Multiple synchronized actuators are needed to insure smooth flap deployment without binding arising from construction tolerances, air loads, and temperature changes.
From a practical standpoint, the chord (the fore and aft dimension) of a Fowler flap is commonly limited to perhaps one-fourth or one-third of the wing chord to minimize actuation problems and drag of the supporting flap tracks. If both the mechanical actuation complexity, and the size and number of external flap tracks could be reduced, it would be feasible and desirable to use flaps with a larger chord. For example, a large flap chord in the range of 50 to 75 percent of the wing chord could provide substantial improvement in low-speed performance of a wing which would still be optimized for high-speed retracted-flap operation.
These goals are achieved by the Fowler-flap support and actuation systems of this invention. A new approach to flap support permits elimination of external midspan flap tracks, and only a single external track is needed. This single track preferably is at the wing root, and can be flush mounted in the fuselage for drag minimization. The support system also solves the problem of binding during flap movement, enabling use of a single simple actuator to deploy a wide-chord Fowler flap.